pgsql: Fix another case of indirectly casting away const.

Fix another case of indirectly casting away const.

Like 8f1791c61, this fixes a case of implicitly casting away
const by not treating the result of strrchr() on a const pointer
as const. This was missed at the time because the machines
reporting those warnings weren't building with --with-llvm.

While here, clean up another infelicity: in the probably-
impossible case that the input string contains only one dot,
this function would call pnstrdup() with a length of -1
and thereby emit a module name equal to the function name.
It seems to me we should emit modname = NULL instead.

Also remove a useless Assert and two redundant assignments.

Back-patch, as 8f1791c61 was, so that users of back branches
don't see this warning when building with late-model gcc.

Reported-by: hubert depesz lubaczewski <depesz(at)depesz(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/aiGNJ89PBqvq2Yyz@depesz.com
Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/96d68502801dcfe7165e3a586bed20979dda51e5

Modified Files
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src/backend/jit/llvm/llvmjit.c | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)